How can I {{#get}} every x post, starting with y?

An example:
I want to get every 3rd post, starting with 1. That would be 1, 4, 7, 10, 13 etc.
I know {#has number=“nth:x”} but it will start with x and not a y defined by me.

I don’t think this would be possible with the GET/HAS helpers, but you could use CSS:

.post {display: none;}    
.post:nth-child(3n+0) {display: block;}

Sadly, this will not solve my use case.